1. Field of the Invention
The present invention relates to the field of digital-to-analog (D/A) converters and, more particularly, to the layout arrangement of current sources in a current-mode D/A converter.
2. Description of Related Art
Current-mode D/A converters have become popular in recent years due to the increase in digital audio and video applications. A schematic circuit diagram of a conventional 8-bit D/A converter is shown in FIG. 7. This D/A converter has two 4-to-15 converters 71 and 72 for converting an 8-bit input code, denoted by I0.about.I7, into a thermal code. The converter 71 converts the most significant bits (hereinafter abbreviated as MSB) 14.about.17 of the input code into 15 MSB outputs M1.about.M15. Also, the converter 72 converts the least significant bits (hereinafter abbreviated as LSB) I0.about.I3 of the input code into 15 LSB outputs L1.about.L15. These outputs M1.about.M15 and L1.about.L15 are referred to as the thermal code, which is characterized by the fact that only one of the outputs changes state when the input code is incremented so as to prevent a sudden change from occurring in output current.
Such a D/A converter further includes 15 MSB current sources 73 and 15 LSB current sources 74. Each of MSB current sources 73 corresponds to one of the MSB outputs M1.about.M15. The output of each MSB current source 73 is connected to a switch 731 which is turned on or off by a corresponding MSB output M1.about.M15, such that the output current from each of the MSB current sources 73 is controlled to be selectively output by the corresponding MSB output M1.about.M15. Similarly, each of LSB current sources 74 corresponds to one of the LSB outputs L1.about.L15. The output of each LSB current source 74 is connected to a switch 741 which is turned on or off by a corresponding LSB output L1.about.L15, such that the output current from each of the LSB current sources 74 is controlled to be selectively output by the corresponding LSB output L1.about.L15. Accordingly, the digital input code I0.about.I7 can be converted into a corresponding analog current I.sub.out for output.
Generally, one LSB current source 74 is implemented by a metal oxide semiconductor (MOS) transistor, and one MSB current source 73 is comprised of 16 MOS transistors for providing a larger output current. Therefore, the circuit layout of the current source transistors for the conventional D/A converter can be shown as in FIG. 8, wherein all the required transistors 81 are arranged in a 15.times.16 array. Each column of the array includes 16 transistors 81 electrically added together to form a MSB current source 73, and fifteen transistors 81 in the rightmost column are provided to be the 15 LSB current sources 74, respectively.
In such a circuit layout, the transistors of the MSB current sources 73 are sequentially arranged from left to right, which may result in producing gradients in the transistor values of the transistors due to process inaccuracies. Therefore, a known DNL (Differential Non-Linearity) error is likely to be caused by the gradients. Such a DNL error negatively affects the accuracy and performance of the DIA converter.
To avoid the above DNL error, U.S. Pat. No. 5,568,145 for a "Current Source Layout Technique to Minimize Deviation" granted to Reynolds is disclosed to have an array of current source cells, wherein the LSB current source cells are physically arranged centrally with respect to the MSB current source cells, and the cells of each MSB current source are electrically added together and physically located along a laterally diagonal path. Although such a circuit layout may eliminate the DNL error caused by the gradients, it is only suitable to an even-bit D/A converter because the number of all the LSB current sources has to be approximately equal the number of the cells of one MSB current source. In other words, it is not applicable to the odd-bit D/A converters, such as 9-bit D/A converters, which are commonly used in audio/video products. Furthermore, because the cells of each MSB current source are added and located in a laterally diagonal manner, the wire connection of the circuit layout is difficult and thus the layout area is increased.
Therefore, it is desired to have an improved layout arrangement of current cells in a D/A converter to overcome the aforementioned drawbacks.